The purpose of this project is the characterization of viruses of mycoplasmas and spiroplasmas. In the latter, new studies including new isolates from ticks, bees, and flowers as well as old strains from citrus, corn, and Drosophila, confirmed by both electron microscopy and cross-plaquing that nearly all strains carry one or more of 3 morphologically distinct viruses resembling bacteriophages. New data on the polyhedral SVC3 indicated host-range differences among virus strains from different sources, showed an adsorption constant of 1.3 times 10 to the ninth power cm3/min, a latent period of 2 hours, a burst size of 26 to 30 plaque forming units, and a K value of homologous antiserum of 880. The double-stranded DNA (MW 13.36 times 10 to the sixth power daltons) was found to be circularly permuted and terminally repetitious, with a GC content of 27 mol. percent; it was cut by 3 restriction endonucleases (EcoR1, Hind III, Hae III) but not by a fourth (BamI). The rod-shaped virus, SVC1, was isolated from, and propagated on, a bee spiroplasma: it appears to contain DNA, and further characterization is in progress.